Frequency detectors are devices that determine frequency and phase information for signals. Frequency detectors are used in communication and power systems as well as consumer electronic devices to aid in synchronizing multiple signals at varying frequencies to a known frequency. A first electrical system may be embodied, for example, as a power grid transmitting electricity with a frequency that may drift over time. A second electrical system may be embodied as a generator for injecting electricity into the power grid. In order to be injected efficiently, however, the frequency of the generated power must be phase aligned with the grid power. Thus, a frequency detector may be used within the generator to detect the shifting power grid frequency and synchronize its generated electricity to the grid frequency for injection.
Zero-crossing detection methods have been used to determine signal frequencies. Zero-crossing detectors may be implemented in either hardware or software systems. A sinusoidal signal may be quantitatively measured using an X-Y plane. The X-axis may represent time and the Y-axis may represent signal amplitude. The signal amplitude may oscillate between a positive peak and a negative peak about the median “zero-level” amplitude for the signal. The zero-level amplitude may be equivalent to system ground. A zero-crossing detector may calculate the rate at which a sinusoidal signal may oscillate about a Y-axis zero-level amplitude over an X-axis time period to determine the signal frequency. For each time the signal crosses the zero-level amplitude, the zero-crossing detector may increment a counter. For example, a sinusoidal signal that crosses the zero-level amplitude Y-axis 120 times in one second may be detected as having a 60 Hz frequency—two crossings may represent a complete signal cycle.
A zero-crossing detector may be implemented by sampling an unknown signal to count each zero-level amplitude crossing of the signal. A limitation of zero-crossing detectors is that they must operate at a high sampling rate in order to count each zero-level signal crossing. This is necessary because zero-crossing detectors compare individual amplitude sample values to an anticipated zero-level amplitude value to determine if the zero-level has been crossed. If the input signal is sampled too slowly, the zero-crossing detector may “miss” the zero-level sample value and thus incorrectly estimate the input signal frequency. The sampling rate must operate several orders of magnitude faster than the signal frequency being detected. A high sampling rate translates to a high operating power for zero-crossing detection methods. Another limitation of zero-crossing detectors is that they count all zero-level amplitudes. If a sinusoidal signal is particularly noisy (e.g., it has a lot of spurious artifacts that distort the sinusoid), a zero-crossing detector is unable to differentiate spurious zero-levels from actual signal zero-levels. Thus, zero-crossing detectors often require input signals to be pre-conditioned to remove such noise. This signal pre-conditioning requires the implementation of additional devices which consume silicon area to remove the signal noise prior to the zero-crossing detection.
Accordingly, there is a need in the art for a frequency detector that conserves power and minimizes silicon area.